1. Field of the Invention
The present invention relates to a control circuit for reducing Total Harmonic Distortion (THD) in the power supply to an electric load.
2. Present State of the Art
In civil and industrial lighting applications, use is known to have been increasingly made of solid state devices, such as LEDs, which require electronic power supplies for proper operation. These power supplies allow galvanic isolation of the light source from the electric network and can supply controlled current to the light source. A number of parameters are used to describe the electric performances of these power supplies, including conversion efficiency, the power factor (PF) on the electric network and the Total Harmonic Distortion (THD) of the current drawn from the electric network.
The Total Harmonic Distortion THD of a signal is the ratio of the sum of the powers of all the harmonics above the fundamental, and the power of the fundamental. The energy transfer efficiency increases as the power factor on the electric network (and hence the electric load systems) becomes closer to one, and the Total Harmonic Distortion on the mains becomes closer to zero.
For instance, in the ideal case of a purely resistive load, supplied with purely sinusoidal AC voltage, there will be a maximum energy transfer efficiency, with a power factor one, and a zero harmonic distortion of the absorbed current.
The electric energy required for lighting is a relevant part of the total electric energy production. Therefore, in electronic power supplies, all the solutions that can improve the power factor and decrease the harmonic distortion in the current drawn therefrom are very important for energy transfer purposes. Hence, the need is increasingly felt of increasing the power factor and decreasing the harmonic distortion of the current drawn from the electric network.
At present, electronic power supplies which are known and recently produced mainly use switching techniques for energy conversion. Such techniques afford high conversion densities and low costs. One or more conversion blocks are usually provided, each optimized for a particular task.
FIG. 1 schematically shows a prior art power supply, in which the network voltage is initially rectified by the diode bridge; then the Power Factor Controller (PFC) allows control of the power factor, reduction of THD, as well as optimization of energy transfer efficiency. Finally, power is supplied to the load (LOAD) through the bulk capacitor (CO) and the converter connected thereto.
A number of active circuits known as “Power Factor Circuits”, which are designed to improve the power factor, are available. The prior art usually includes buck, boost, buck/boost and flyback topologies for this conversion stage.
The most common circuits for low-power applications (which are typical for LED lighting applications) are the one as shown in FIG. 1 (Transition Mode Boost PWM circuits—fixed ON time, variable frequency) and its insulated derivative, as shown in FIG. 2 (bulkless flyback circuits).
Active circuits draw a sinusoidal current in phase with the voltage from the mains, and load a capacitor to a predetermined voltage through a switching circuit controlled by an integrated PFC controller. In order to accomplish this task, the circuits designed for this type of application use information proportional to the mains voltage (e.g.,. VL in FIGS. 1, 2) and the voltage proportional to the current in the switching circuit (VS in FIGS. 1, 2). If the “Power Factor Circuit” operates with a predetermined input voltage within a restricted range, the VL and VS signals may be optimized to obtain optimal PF (Power Factor) and TDH (Total Harmonic Distortion) values.
If the “Power Factor Circuit” operates over a wide input voltage range, e.g., 90 VAC-305 VAC, the VL and VS signals are not easily optimized throughout the input voltage range. The power factor and the total harmonic distortion are strongly affected by this phenomenon, whereby no satisfactory result can be obtained (in terms of power factor maximization and total harmonic distortion minimization) over the entire range of possible input voltages.
This adds operational restrictions in prior art circuits because, in response to considerable supply voltage changes, they cannot ensure optimal power supply to the load.